"Rust is a shifty, changing, constantly evolving enemy. We can never lower our guard. We must fight rust by all means open to science."
—E. C. Stakman (1937)
Stem rust has been the cause of devastating epidemics throughout history, because under the right conditions for disease development it can cause 50-100% yield loss. Resistance to particular rust “races” can be provided by single resistance genes in the plant that recognize specific rust effectors, but the fungus quickly evolves to evade this recognition. In the 1950’s, new wheat strains resistant to stem rust were developed and distributed throughout the world as part of the Green Revolution. However, in 1999 a new strain of stem rust arose in Uganda (termed Ug99 for its country of origin) that overcame the resistance of existing wheat lines. Since then, Ug99 has been spreading through East Africa into the Middle East.
Two other important wheat rust pathogens, stripe (yellow) rust and leaf rust, also cause significant annual losses wherever wheat is grown.
While any one resistance gene can be easily overcome, a combination of multiple genes (a “stack) requires multiple changes in the pathogen to occur at once. A stack of 3-5 effective resistance genes has a very small chance of being overcome. Recently, several resistance genes against Ug99 have been cloned by our collaborators and others, and many more are in the process of being isolated. Our goal is to combine several isolated resistance genes and work with partners to introduce them together into preferred varieties of wheat. A similar strategy would be effective against stripe and leaf rusts, and 2Blades is also working with collaborators to identify resistance genes effective against these diseases.
Pathogen recognition by resistance genes
Pathogens including rusts introduce a variety of effectors into the host cell to alter the host metabolism to benefit the pathogen or to suppress host defense responses, and many plant resistance proteins recognize those effectors or their secondary effects. If an effector is recognized by a resistance protein in the plant, then the infection can be stopped or slowed, and the plant will be resistant to that pathogen. However, if an effector can be changed or lost without affecting the virulence of the pathogen, then new pathogen races lacking it can be selected. The majority of resistance genes against wheat rusts are race-specific, meaning that they recognize some races of the pathogen but not others, based on the effectors present in each race. Most race-specific resistance genes against rusts are of the NB-LRR type. 2Blades has pioneered a technique for rapidly isolating these NB-LRR resistance genes.
A second type of resistance gene found in wheat is known as an adult plant resistance (APR) gene. These genes are not specific to a given race of rust, and are often effective against multiple rust species. While resistance mediated by APR genes is generally only partial, it can be significant. Stacking both race-specific and APR genes may provide even more durability.
Maintaining resistance to rust fungi is challenging because of the nature of the pathogen. When growing on wheat, stem rust produces 100 billion spores per hectare, or around 19 kg/ha of spores! These spores can be spread by wind over long distances. The spores produced on wheat plants are asexual, meaning that they are genetically identical to the parent unless a mutation occurs. However, given the very large number of spores produced, there is a high chance of any given single mutation arising. For further complexity, stem rust also has a sexual reproduction cycle on the alternate host, barberry, allowing new combinations of effectors to be generated. To learn more about stem rust, see this video by the Borlaug Global Rust Initiative.
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